Bring the milk to the boil with butter and salt. Meanwhile, mix the starch with the water, cocoa and sugar.
Remove the milk from the stove and stir in the cocoa-sugar mixture. Put back on the stove, bring to the boil and simmer for a minute. Pour into a dish rinsed with cold water and put in a cool place.
I put everything in small pudding molds, so the kiddies don`t argue about who gets more or less.
Another tip: You can avoid the pudding skin that forms when it cools down by placing cling film directly on the pudding.